1. Field of the Invention
The present invention relates to techniques for reducing the migration of radioactive materials from a nuclear reactor to a steam and turbine system.
2. Description of the Related Art
In a nuclear power plant, the reduction of exposure during operation and regular inspection is important. Various materials have been proposed, water quality control measures have been taken and the improvement of purifying facilities have been made principally for the reduction of the cobalt-60 concentration of reactor water and the migration of radioactive materials to the water systems of nuclear reactors. However, any measures to reduce the migration of radioactive materials to the main steam line and turbine system have not been taken.
Modes of making radioactive materials migrate to a steam system of a present nuclear power plant system and the ratio between the radioactive material carrying effects of the modes will be explained with reference to FIGS. 1 and 2 and problems to be solved will be explained.
Referring to FIG. 1, part of radioactive materials produced by a reactor core 10 contained in a reactor pressure vessel (hereinafter referred to simply as “pressure vessel”) 9 is removed by a reactor water purifying system 5. Most part of the radioactive materials adheres to devices placed in the pressure vessel 9 through which saturated water is circulated, the inner surface of the pressure vessel 9 and pipes extending on the upper side of the reactor water purifying system 5. Very small part of the radioactive materials dissolved in the reactor water in ions or molecules has a partial vapor pressure and volatilizes together with steam.
On the other hand, a steam separator 4 and a dryer 3 remove most part of liquid drops. Very small part, i.e., 0.1% or below, of liquid drops migrates in small particles through a valve 1 placed at the inlet of the turbine system into a steam turbine and contaminates the steam turbine system and the associated parts. Recently, the temperature of the nuclear reactor is lowered rapidly when the nuclear reactor is shut down. When thus shutting down the nuclear reactor, water having a high radioactive concentration and discharged from the reactor water purifying system 5 is sprayed by a head spray nozzle 6 and, consequently, radioactive materials contained in the sprayed liquid drops migrate to the steam system.
The behavior of liquid drops in a dryer (steam dryer) 3 closely related to the migration of liquid drops to the steam system will be described with reference to FIG. 2. Steam containing liquid drops in a wetness of 10% or below and passed through the steam separator 4 shown in FIG. 1 is dispersed by steam dispersing openings 20, flows through spaces between corrugated plates 22 and an upper part of the pressure vessel 9 into a line 11. Whereas the steam flows along the surfaces of the corrugated plates 22, liquid drops having large mass collide against the surfaces of the corrugated plates 22 and are caught by the corrugated plates 22. The liquid drops thus caught by the corrugated plates 22 flows down along the corrugated plates 22 into a drain pan 24, and returned into the reactor water system through a drain pipe 25.
If the steam containing liquid drops flows at a high velocity through the spaces between the corrugated plates 22; that is, if time for which the steam stays in the spaces between the corrugated plates 22 is short, minute liquid drops flow through the spaces between the corrugated plates 22 without colliding against the corrugated plates 22. Furthermore, the steam flowing at a high velocity through the spaces between the corrugated plates 22 separates the liquid drops and radioactive materials, which have been collided against and adhering to the corrugated plates 22, off the corrugated plates 22 and carries the same away to the steam system.
Thus, the ratio of migration of radioactive materials to the steam system in an actual nuclear power plant is greater than that calculated on the basis of the gas-liquid distribution ratio of the radioactive materials dissolved in water. Radioactive materials migrate to the steam system in the following three modes; (i) a first migration mode in which radioactive materials dissolved in the reactor water evaporate and migrate into the steam system, (ii) a second migration mode in which liquid drops collided against the components of a device, such as a dryer, and caught by the components of the device are separated from the components of the device in liquid drops or radioactive materials dried and adhering to the components of the device are separated from the components of the device, and the separated liquid drops or the separated radioactive materials are carried into the steam system and (iii) a third migration mode in which liquid drops sprayed by a head spray migrate to the steam system. The ratio between the respective radioactive material carrying effects of those modes is 1:3:1.
Recently, the enhancement of power, temperature capacity and pressure capacity without changing the sizes of devices has been desired from the economical point of view. However, in view of the forgoing problems, it can be readily conjectured that the ability to separate liquid drops from steam of the steam separator and the dryer will become insufficient and the migration of radioactive materials to the steam system will increase when the power capacity of the nuclear reactor is increased. If the temperature capacity and pressure capacity are further enhanced, a supercritical state will result. In the supercritical state, all the particles of radioactive materials contained in supercritical water or all the radioactive materials dissolved in supercritical water migrate to the steam system unless a radioactive material separating and removing apparatus is used. Therefore a high-temperature water purifying apparatus is one of the most important apparatuses of the supercritical reactor. In this specification, the term “high-temperature water purifying apparatus” is used to denote an apparatus capable of being used under a high-temperature condition for separating and removing radioactive materials from water or steam.
Various high-temperature water purifying apparatuses that do not entail heat loss have been proposed. However, none of the previously proposed high-temperature water purifying apparatuses is able to avoid performance degradation due to rise in differential pressure caused by contamination by collected corrosion products and organic substances used by the turbine system, and changes in the shape of small holes caused by the volume expansion due to the dissolution, corrosion or oxidation of a filter aid by the chemical instability of high-temperature water and those high-temperature water purifying apparatuses have a short life. Furthermore, those high-temperature water purifying apparatuses have a low trapping capacity and their purifying ability deteriorates in a short time. Consequently, those high-temperature water purifying apparatuses have not been applied to practical uses yet. (Refer to “Filter Guidebook for Pall-Generator”, Nihon Pall Ltd., P.8.)